L.W. Braile,

September, 2000

Objective:Demonstrate plate tectonic
principles, plate boundary interactions and the geometry and relative motions
of faulting of geologic layers using 3-D foam models.The foam models aid in visualization and
understanding of plate motions and faulting because the models are
three-dimensional, concrete rather than abstract descriptions or diagrams, can
be manipulated by the instructor and the students, and the models can show the
motions of the plates and faults through time in addition to the
three-dimensional configuration of the plates or layers.The fault and plate boundary models shown
here illustrate relatively simple motions and geologic structures.Although these models are accurate
representations of real Earth faulting and plate tectonic structures and
motions, the spherical shape of the Earth and the complexity of geological
features caused by varying rock types and rock properties and geological
development over many millions or hundreds of millions of years, result in
significant complexity and variability of actual fault systems and plate
tectonic boundaries.

1.Faulting
and Plate Boundaries - Prepare foam block models as shown in Figure 1A.The cardboard (cut from manila folders or
thin poster board) attached to both faces of the fault plane allows the blocks
to slip easily along the fault as forces are applied to the blocks.Use the block models to demonstrate
extensional (normal) faulting as the two outer blocks are moved apart as shown
in Figure 1B.This procedure is best
performed by holding the blocks “in the air” in front of you, supporting the
model by the two outer blocks, rather than on a table.Note that as the two outer blocks are moved
apart, the inner block drops downward or “subsides.”This relationship between extensional motion
of geologic layers and downdropped fault blocks (graben or rift valley if the
downdropped block is bounded on both sides by normal faults, as in this block
model) produces normal faulting (Figure 2) and also represents the extensional
motion and resultant rift development associated with divergent plate boundaries
(Table 1).Examples of divergent plate
boundaries, where extensional faulting is prominent, are the mid-ocean ridge
system in which a narrow rift or graben (downdropped fault block) is commonly
observed along the highest part of the ridge (see section 2 below) and the East
African Rift in which extension has been occurring in the continental
lithosphere for about 30 million years and the resulting rift system of normal
faults is beginning to break apart the continent.In a plate-tectonic-related, but not plate
boundary environment, the Basin and Range area of the Western United States
displays a prominent topographic signature of extensional faulting with many
adjacent downdropped fault blocks or grabens (the topographic “high” areas
between the grabens are called horsts; see IRIS poster on western US seismicity
and topography).

To demonstrate compressional motion and resulting reverse
(also called thrust) faults (Figure 2), hold the foam block models as described
above and then move the two outer blocks together as in Figure 1C.The inner block will be thrust upwards
producing reverse faults and an uplifted block.In a plate tectonic setting, such compressional motion is associated
with convergent plate boundaries (Table 1) where two lithospheric plates are
moving together or colliding (see also section 3 below).Not surprisingly, these convergent zones are
associated with mountain ranges (Himalayas, Alps, Andes,
Cascades, etc.).

To demonstrate horizontal slip or strike-slip fault motion,
prepare foam blocks as shown in Figure 1D.Moving the blocks horizontally on a tabletop, as shown in Figure 1E,
demonstrates strike-slip (Figure 2) or horizontal slip fault motion.This motion along a plate boundary is also
called transform (Table 1).The San Andreas fault zone is a system of strike-slip faults
which form the transform plate boundary at the western edge of the North
American Plate.Transform faults also
occur as oceanic fracture zones between segments of the mid-ocean ridge
spreading zones (see ocean bathymetry map in a world atlas, such as the
National Geographic World Atlas, or view ocean bathymetry on the Internet
at:http://www.ngdc.noaa.gov/mgg/announcements/images_predict.HTML; click on
one of the regions containing a mid-ocean ridge to see details of ridge crest
and transform fault topography of the ocean floor).

2.Divergent Plate Boundary and Sea Floor Spreading -
Prepare the foam pieces that represent the oceanic lithosphere at a spreading
center (mid-ocean ridge divergent plate boundary) as shown in Figure 3A.Cut 10 one cm by 20 cm strips of the
closed-cell foam material.Color half of
the strips black with the felt pen and label all of the foam pieces as shown in
Figure 3A.Construct a “ridge”
(optional) to form the base for the sea floor spreading model.The ridge surface represents the top of the
asthenosphere in the upper mantle and the foam layer above the base is the
oceanic lithosphere - typically about 50-100 km thick in the Earth.The base also provides a mid-ocean ridge
topography in which the spreading and extension occurs along the narrow rift
zone along the ridge crest.

To demonstrate the concepts of a divergent plate boundary
and mid-ocean ridge spreading centers, begin by placing the two 20 x 20 cm foam
pieces on the base (Figure 3B) with one edge adjoined at the ridge crest and
the arrows on the foam pieces pointing outward (Figure 3A).These squares will represent oceanic
lithosphere at a time five million years ago and thus contain oceanic crust
(the upper layer of the lithosphere) that is 5 million years old and
older.Slide the two foam squares away
from each other about 2 cm (this process represents the passage of time and the
extension of the lithosphere in the region of the ridge crest, and rift valley,
by plate tectonic motions which are typically a few centimeters per year,
equivalent to a few tens of km per million years) and place the two strips
labeled 4 million years in the space that is created.Attach one strip to each edge of the squares
using pins.In the real mid-ocean ridge,
a void space or opening between the plates created by the spreading process,
would not actually develop.Instead, as
extension occurs, volcanic and igneous intrusion processes will relatively
continuously fill in the extended lithosphere, in the process creating new
lithosphere.Because the oceanic crustal
layer in this new lithosphere is formed from igneous (volcanic and intrusive)
processes, it cools from a liquid and the rocks acquire a remanent magnetic
direction that is consistent with the Earth’s magnetic field direction at that
time.Because the Earth’s magnetic field
occasionally reverses its polarity (north and south magnetic poles switch), the
lithosphere created at mid-ocean ridges displays “stripes” of normal and
reversed magnetic polarity crust approximately parallel to the ridge
crest.Additional information on these
magnetic stripes and mid-ocean ridge processes can be found in “This Dynamic
Earth”.The igneous rocks which are
formed at the ridge crest can also be “dated” using radiometric dating of rock
samples to determine the age of the volcanism and intrusion.

Continue to extend the two plates away from each other at
the ridge crest and add the new pieces of lithosphere (attach with pins) which
are labeled in decreasing age (3, 2, 1 and 0 million years old).When you are finished, the mid-ocean ridge
divergent plate boundary and adjacent lithosphere should look like the diagram
shown in Figure 3A and represent a modern (zero million years old) mid-ocean
ridge spreading center.Note that the
youngest rocks are in the center, along the ridge crest, and the rocks are
progressively older (to 4 million years old in the strips and 5 million years
old and older in the lithosphere represented by the squares of foam) away from
the ridge crest.

3.Convergent Plate Boundary and Subduction -
Arrange two tables of identical height to be next to each other and about 30 cm
apart as shown in Figure 4.Place the
two pieces of one-inch thick foam on the tables and begin to move one piece of
foam (the one without the cardboard edge) toward the other and allow it to be
“thrust” beneath the other piece of foam.The foam pieces represent two lithospheric plates.As the convergence continues, the underthrust
plate will form a subducted slab of lithosphere (extending to at least 600 km
into the mantle in the Earth) as shown in Figure 4.Earthquakes commonly occur along the length
of the subducted slab and compressional structures (folds and faults) are often
associated with the compressional zone near the colliding plates.The subducted lithosphere consists of
relatively low-melting-point rocks (sediments and oceanic crust form the upper
layers of the oceanic lithosphere) which can melt at depths of 100-150 km as the
slab is subducted into the mantle.These
molten materials can then ascend through the overlying mantle and crust and
form volcanoes which are often situated in a linear chain or arc about 100-200
km away from the collision zone.A deep
ocean trench also forms above the point of convergence of the two plates as the
oceanic lithosphere is bent downwards by the collision.

4.Transform or Strike-Slip Plate Boundaries and Elastic Rebound - Use
a razor-blade knife to make the foam “plate” models shown in Figure 5.The foam is 1.25 cm (1/2”) thick closed-cell
foam often used for “sleeping pads” for camping.It is available at camping supply stores and
Wal-Mart and Target.The foam pieces can
be used on a table top or on an overhead projector (the slits cut in the foam
allow the 10 cm long tabs which bend to be seen projected onto a screen).By continuously sliding the two plates past
each other with the “tab” edges touching (Figure 5), the foam pieces represent
lithospheric plates and the “zone” where the plates touch is a strike-slip
(transform) fault.Note that as the
plates move slowly with respect to each other (just as Earth’s lithospheric
plates move at speeds of centimeters per year), the area of the plates adjacent
to the fault (the tabs) becomes progressively bent (deformed), storing elastic
energy.As the process continues, some
parts of the fault zone will “slip” releasing some of the stored elastic
energy.This slip occurs when the stored
elastic energy (bending of the tabs) results in a force along the fault which
exceeds the frictional strength of the tabs that are in contact.Sometimes, only small segments of the fault
zone (one or two tabs) will slip, representing a small earthquake.At other times, a larger segment of the fault
will slip, representing a larger earthquake.Note that although the plate motions are slow and continuous, the slip
along the fault is rapid (in the Earth, taking place in a fraction of a second
to a few seconds) and discontinuous.The
motions and processes illustrated by the foam model effectively demonstrates
the processes which occur in actual fault zones and the concept of the elastic
rebound theory (Bolt, 1993).A brief
segment during the beginning of the video “Earthquake Country” illustrates a
similar “stick-slip” motion using a model made of rubber strips.

Extensions,
Connections, Enrichment:

1.Good preparatory lessons for these activities are studies of
elasticity (a spring and masses can be used to demonstrate the two fundamental
characteristics of elasticity - the stretching is proportional to the force
(suspended mass) and the existence of the “restoring force” (elastic energy is
stored) in that the spring returns to its original length as the force (mass)
is removed), and seismic waves which are generated as the fault slips.

2.The stick-slip process is well illustrated in a segment of the
NOVA video “Killer Quake” in which USGS geophysicist Dr. Ross Stein
demonstrates this process using a brick which is pulled over a rough surface
(sandpaper) using an elastic cord (bungy cord).An experiment using this same procedure is described in “Seismic
Sleuths” (AGU/FEMA).

3.Additional information on plate tectonics is available in Bolt
(1993), Ernst (1991), Simkin et al. (1994), the TASA CD “Plate Tectonics,”
“This Dynamic Earth,” and nearly any secondary school or college level geology
textbook.Elastic rebound is well
illustrated in Lutgens and Tarbuck (1996), Bolt (1993) and the TASA CD.A color map of the Earth’s plates is
available on the Internet at:http://www.geo.arizona.edu/saso/Education/Plates.An excellent description of plate tectonics
can be found at:http://pubs.usgs.gov/publications/text/understanding.html.

4.An additional plate tectonic activity is the EPIcenter lesson
plan “Voyage Through Time - A Plate Tectonics Flip Book” in which continental
drift during the past 190 million years - a consequence of plate tectonics - is
effectively illustrated; and Plate Puzzle which uses the "This Dynamic
Planet" map.

6.A leading theory explaining why the Earth’s plates move
is convection currents in the Earth’s mantle.The interior structure of the Earth is described in Bolt (1993) and is
the subject of the EPIcenter activity “Earth’s Interior Structure.”Good activities illustrating convection are
contained in the GEMs guide “Convection - A Current Event” (Gould, 1988), or
“Tremor Troop” (NSTA/FEMA).

Figure 1.Foam (soft, open cell
foam used for mattresses) blocks for demonstrating faults (normal, reverse and
strike-slip) and motions at plate boundaries (divergent and extensional motion;
convergent and compressional motion; transform and horizontal slip
motion).Large arrows show direction of
force or plate motion.Half-arrows along
faults show direction of relative motion along the fault plane.Shaded area is red felt pen reference
line.A.Foam block with 45° angle cuts (cardboard, cut
from manila folders, attached to angled faces with rubber cement) and reference
line drawn on the side of the blocks with a felt pen.B.Response of model to extension.C.Response of model to
compression.D.Foam blocks used to demonstrate strike-slip
motion.Cardboard is attached to the two
faces (as shown in Figure) using rubber cement.Reference lines and arrows are drawn on the top of the foam blocks using
a felt pen.E.Response of model to horizontal slip motion.

Figure 2.Block
diagrams illustrating types of geological faults with resulting offsets of
layers.Half-arrows show relative motion
of the blocks along the fault plane.

Figure 3.Foam
pieces for demonstrating divergent plate boundaries and a mid-ocean ridge
spreading center.Cut out pieces with
razor blade knife and straight-edge.A.Top view of foam blocks after assembly (see
text) representing 5 million years of extension at the ridge crest and
generation of new lithosphere by magmatic (igneous) processes.Numbers are ages in millions of years.In the real Earth, the time periods of normal
(shaded) and reversed polarity would not be of equal duration (one million
years in this simulation) and thus the ‘stripes” would be of varying
widths.B.Side view showing foam pieces on top of
styrofoam base (two pieces, each 20 cm x 30 cm) which creates slopes
representing the mid-ocean ridge.Attach
styrofoam with pins to foam piece (2 cm x 20 cm) used to create slope.

Figure 4.Foam
(soft, open cell foam) pieces (each piece is 50 cm by 15 cm by 2.5 cm (1 in)
thick) used to demonstrate convergent plate motions and subduction.Edge of one of the foam pieces is cut at a 45
angle and lined with cardboard (manila folder material), using rubber cement to
attach the cardboard to the foam.

Figure 5.Foam pieces used to
demonstrate strike-slip faulting, elastic rebound theory, and slipping along
the fault plane during earthquakes.Cut
out slits with razor blade knife and straight-edge.

Table
1.Faults, Plate Boundaries and Relative
Motions*

Relative Motion of Layers or Plates

Fault Names

Plate Boundary
Descriptions

Related Tectonic and Geologic Features

Extension

Normal

Divergent (extensional, moving apart, spreading,
construction - because new lithosphere is generated in the extended zone)

*Many terms and geological “jargon” are associated with
faults and plate boundaries.While these
terms are useful to Earth scientists and are included here and in the
accompanying text for completeness, the most important concepts such as
extension, moving apart, downdropped blocks, etc., can be discussed and
understood without unnecessary jargon.Additional information on the terms and concepts used here can be found
in virtually any introductory geology textbook or in the USGS booklet “This
Dynamic Earth - The Story of Plate Tectonics.”